1. Field of the Invention
The present invention relates to an improved programming device for programmable sequence controllers (hereinafter abbreviated as "sequencers") for controlling various types of industrial machinery such as, for example, machine tools. More particularly, the present invention relates to an improved programming device which is capable of easily preparing a sequence control program used in testing or maintaining one or several sequencers of those sequencers to be interconnected in a link system.
2. Discussion of the Prior Art
Heretofore, a system has been known wherein a plurality of sequencers are connected to a link line and wherein a certain sequencer is controlled in a relation interlocked with another sequencer by reference to the states of input and output (hereafter, simply as "I/O") elements belonging to such another sequencer.
In such a system, each sequencer transmits the states of its I/O elements inquired for from another sequencer to the same via the link line. On the other hand, the another sequencer receiving the states of the I/O elements stores the states in its link element memory which is accessible like a memory storing the states of those I/O elements belonging to the another sequencer itself. Therefore, each sequencer makes access to its link element memory when, in accordance with a sequence control program, executing the inquiry for the states of the I/O elements belonging to another sequencer.
However, the system is not necessarily operated with all the sequencers in the system being interconnected by the link line. For example, it is often the case that the system is operated with a certain sequencer not being interconnected for some reason, or that the operating states of the system are verified or checked with a certain sequencer being not linked to the system for the purpose of system maintenance or trial system operation or for the purpose of partially operating the system in the course of building up the system.
In such case, the states of the I/O elements belonging to a sequencer which is not being interconnected with the link line or not being supplied with electric power, cannot be transmitted to any other sequencers through the link line, so that the states of those addresses of the link element memory which are assigned to such non-connected or off-powered sequencer are uncertain for such any other sequencers. However, the sequence control program for each sequencer is prepared as if all the sequencers were interconnected. Because each sequencer makes access to the addresses of its link element memory assigned to the non-connected or off-powered sequencer like those addresses of its memory storing the states of I/O elements belonging to itself and because the states of the addresses assigned to the non-connected or off-powered sequencer are uncertain, such each sequencer cannot execute the sequence control program as designed therefor.
Therefore, where the system is operated without a certain sequencer therewithin being interconnected, it is necessary for the operator to temporarily modify the sequence control programs of those sequencers which inquire for the states of the I/O elements belonging to the non-connected or off-powered sequencer. After the trial operation or maintenance of the system, the modified sequence control programs have to be restored as they were when the system is subsequently to be operated with all the sequencers within the system being in operation. The restoration of the modified programs requires great efforts and much time. Although other measures may be taken for temporarily operating the system without modifying the sequence control programs, such other measure require temporarily changing the wirings corresponding to those I/O elements for which inquires are made from another sequencer and which are of the sequencer being not linked, in order to fix the states of those I/O elements.
Further, in a system wherein a plurality of sequencers are not linked, namely in that wherein a sole sequencer is used, it may be the case or required to fix certain I/O elements to either ON or OFF state for maintenance or test purpose. In such a case, an original sequence control program in which any modification is not made that makes a certain I/O element assumed as if it is fixed to either of ON or OFF state, must be maintained for normal system operation. In addition, where certain I/O elements are set to the fixed states, unless such is visibly displayed for the operator to understand, difficulties may arise in analyzing the operations according to the sequence control program.